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Abstract

Transmission near-infrared (NIR) measurements of a 1 mm thick aspirin disk were made at
different positions as it was moved through a stack of eight 0.5 mm thick disks of
microcrystalline cellulose (Avicel). The magnitude of the first derivative of absorbance for
the aspirin interlayer at 8934 cm−1 was lower when the disk was placed at the top
or bottom of the stack of Avicel disks, with the largest signal observed when the aspirin was
positioned at the central positions. The variation in signal with depth is consistent with
that observed previously for transmission Raman spectrometry. In both cases, the trend
observed can be attributed to lower photon density at the air-sample interface, relative to
the center of the sample, owing to loss of photons to the air. This results in a reduction in
the number of photons absorbed or Raman photons generated and subsequently detected when the
interlayer occupies a near-surface position.

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